Design Note 13 Issue 2 June 1995 High Efficiency Hybrid DC-DC Converter + 12V 0.1uF Output (2W) 0V W1 28T W2 W3 T1 10T 10T + 5V 0.1uF 560 Input 560 W4 ZTX 449 4T ZTX 449 0V To meet the growing requirement of DC-DC converters that fit into 0.6 inch pitch DIL IC packages (for LAN hub power supplies for example), efficient but low cost hybrid circuits must be used. Using low switching loss bipolar transistors from Zetex, converters such as the cir cuit s hown here can be constructed on small substrates giving p o w e r d e n s i t i e s o f o v e r 1 . 7 W / cc . Operating at over 80kHz the toroidal transformer and ceramic decoupling capacitor volumes are minimised, while the consistency of the hFE process provides a balanced switching performance with no worry of low tempera ture start-up or transistor induced shoot-through. The ZTX449 devices, possessing a saturation voltage of only 150mV at 0.5A give the 2W converter an efficiency of over 77%, thus keeping power losses and internal heating at a low level. Maximum operating temperature is ultimately limited by the magnetics but the standard industrial range of -20°C to +85°C is easily covered. Other devices are available that exhibit higher gain and/or higher voltage ratings for other supply voltage options. DN13-1